What you are seeing is an Embed, and it only works for certain sites (see list) and only when the URL is on a line by itself. You can try adding a space in front or some other markup so that it is not just the URL on its own line.
I can think of 4 different ways to do this (includes @joyously suggestion).
1. Don’t put the URL on a line by itself.
2. Disable the link using custom CSS.
3. Disable the link by wrapping it with <code></code> tags.
4. Disable the link using a custom HTML block and wrap it with <pre></pre> tags.